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Rainy Season: The Rainy Season in India (2548 Words)

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The rainy season in India starts with the onset of the southwest monsoon in June and continues till the middle of September!

This is also called the monsoon season, the southwest monsoon season, the wet season and the hot-wet season. The weather conditions all over the country change with the onset of the monsoon winds. High heat, high humidity, extensive clouding and several spells of moderate to heavy rain with strong surface winds are the chief characteristics of this season.

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(a) Temperature:

There is a significant fall in temperature with the beginning of rainy season. The June temperature in south India is 3° to 6°C lower than the May temperature.

Similarly July temperature in northwest India is 2° to 3°C lower than the June temperature. But once the temperature falls from its dry summer level, it remains more or less uniform throughout the rainy season. However, the temperature rises again in September with the cessation of rains and secondary maximum temperature period is experienced all over the country.

Also there is rise in temperature whenever there is break in the monsoons and rainfall does not occur for a number of days. Night temperatures are more uniform than the day temperatures. The diurnal range of temperature is small due to clouds and rains. It ranges between 4°C and 8°C when the monsoon is fully established.

The highest temperatures of over 32°C are experienced in the Thar Desert of Rajasthan. At places especially west of the Aravali the temperature may be as high as 38° to 40°C. This is due to lack of clouds and the predominance of continental airmass. The remaining parts of northwest India also have temperatures above 30°C.

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The temperatures are quite low over the Western Ghats due to high elevation and also due to heavy rainfall, but the rain-shadow area is comparatively wanner on account of low elevation and scanty rainfall. The coastal areas of Tamil Nadu and adjoining parts of Andhra Pradesh have temperatures above 30°C because they receive little rainfall during this season.

(b) Pressure and Winds:

The temperatures in northwest India are still very high as a result of which low pressure conditions prevail there. The most conspicuous feature of the surface pressure distribution during this season is an elongated trough across the Ganga basin right upto the head of Bay of Bengal. This is called the monsoon trough.

There are frequent changes in its location and intensity depending upon the weather conditions. The atmospheric pressure in most parts of north India is less than 1,000 mb. It increases steadily southwards where it ranges between 1,008 mb. and 1,010 mb. The isobar of 1,009 mb. crosses parts of Kerala and Tamil Nadu besides Arabian Sea and the Bay of Bengal.

Under the influence of the above mentioned pressure distribution, winds blow in a southwest to northeast direction from Arabian Sea and Bay of Bengal. They maintain this direction throughout peninsular India. But their direction undergoes a change in Indo-Gangetic plain where they move from east to west.

(c) Rainfall:

India’s three fourths of the total annual rainfall is received during this season. In some areas it is much more than this average. For example, the average rainfall over the plains of India in this season is about 92 cm, or about 87 per cent whereas during the remaining part of the year only 14 cm of rainfall occurs. This season has the maximum number of rainy days as a result of which it is called the ‘wet season’.

Rainfall during this season all over the country is caused by southwest monsoons coming from the Indian Ocean. Figure 5.18 shows isolines of normal dates of arrival of the monsoons in different parts of the country. It is clear from this map that the normal date of the arrival of the monsoon is 20th May in Andaman and Nicobar Islands.

It is worth mentioning that the advance of the monsoon is much more in the Bay of Bengal than in the Arabian Sea. This is evident from the pronounced curve shown by isolines of the monsoon onset in figure 5.18. The monsoon current advances to nearly 20°N latitude in Bay of Bengal by the third week of May, when it is still south of Kerala at about 7°N latitude in the Arabian Sea.

The normal date of onset of the southwest monsoon over Kerala i.e. the first place of entry in the mainland of India is 1st June. The monsoons advance with startling suddenness accompanied with a lot of thunder, lightning and heavy downpour. This sudden onset of rain is termed as monsoon burst. Although the normal date of onset of the southwest monsoon on the southern tip of the peninsula is 1st June, the actual onset may be earlier or later than this date. On 60% occasions, the onset occurs between 29th May and 7th June. The earliest onset was on 11th May in 1918 and 1955, while the most delayed onset was on 18th June in 1972. Satellite imagery is used to identify the advance of the monsoon on a day to day basis.

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The progress of the monsoon winds beyond south Kerala is in the form of two branches viz. the Arabian Sea branch and the Bay of Bengal branch. The Arabian Sea branch gradually advances northwards. It reaches Mumbai by 10th June and spreads over Saurashtra-Kuchchh and central parts of the country by 15th June.

The progress of the Bay of Bengal branch is no less spectacular. It spreads rather rapidly over most of Assam. The normal date of its arrival at Kolkata is 7th June. On reaching the foothills of the Himalayas the Bay branch is deflected westward by the Himalayan barrier and it advances up the Gangetic plain.

The two branches meet roughly along the line running through Agra and Ferozepur and merge with each other to form a single current. The combined current gradually extends to west Uttar Pradesh, Haryana, Punjab, Rajasthan and finally to Himachal Pradesh and Kashmir. By the end of June the monsoon is usually established over most parts of the country.

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By middle July, the monsoon extends into Kashmir and the remaining parts of the country, but only as a feeble current because, by this time, it has shed most of its moisture. It is often difficult to say whether the Arabian Sea branch or the deflected Bay of Bengal branch will be the first to arrive.

For example, at a place like Delhi, the first showers are sometimes brought by the Bay of Bengal branch from the east but on a number of other occasions it is the Arabian Sea branch which brings the first monsoon rain from the south. It is interesting to note that the Arabian Sea branch of the monsoon is much more powerful than the Bay of Bengal branch for two reasons: (i) The Arabian Sea is larger than the Bay of Bengal, and (ii) the entire Arabian Sea current advances towards India, whereas only a part of the Bay of Bengal current enters India, the remainder proceeding to Myanmar, Thailand and Malaysia.

The Arabian Sea branch of the southwest monsoons is divided into three distinct streams on arriving in the mainland of India.

The first stream impinges on the west coast of India and gives extremely heavy rainfall of over 250 cm particularly between 10° to 20°N. latitudes where the impinge is perpendicular to the direction of the Western Ghats. The Western Ghats rise abruptly like a wall from the western coastal plains more or less parallel to the coastline.

This wall like mountain range forces the moisture had laden southwest monsoons from the Arabian Sea to ascend the slope thereby giving heavy rainfall in the west coastal plain and still heavier rainfall on the western slopes of the Western Ghats, i.e., the windward side of the mountain range. But these winds have to descend the slope after crossing the crest of the ghats. In the process their temperature rises and their humidity decreases. Therefore, they cause little rainfall and the area east of the Ghats is called the ‘leeward side’ or the ‘rain shadow region’.

Thus while Mumbai on the west coast records about 190 cm, Khandala about 50 km east a bit below the crest gets 460 cm and Pune about 160 km away from Mumbai on the leeward side receives only 50 cm rainfall during the monsoon season. This phenomenon is observed almost all along the Western Ghats.

The crest of the Western Ghats receives about 400 to 500 cm annual rainfall which is drastically reduced to about 30-50 cm within a distance of 80-100 km leeward from the crest. This speaks volumes of orographic control of the monsoon rainfall. There is a narrow belt of marked aridity on the immediate leeward side of the Western Ghats. But once it is passed, the air starts rising again and the amount of rainfall increases further east.

The second stream enters Narmada-Tapi troughs and reaches central India. It does not cause much rain near the coast due to the absence of any major orographic obstacle there. Nagpur receives about 60 cm rainfall from this stream.

The third stream moves in a north-easterly direction parallel to the Aravali Range. Since the orientation of the Aravali Range is parallel to the direction of the prevailing monsoon winds, it does not offer major obstacle in the way of the winds and these winds move further without causing much rainfall.

Consequently the whole of Rajasthan is a desert area. However, some orographic effect is discernible here as the south-eastern edge of the Aravali Range comes in the way of the monsoon winds and receives reasonably good rainfall. Mt. Abu gets about 170 cm rainfall while the surrounding plains have only 60 to 80 cm rainfall.

The Bay of Bengal Branch of the southwest monsoon is divided into two distinct streams:

The first stream crosses the Ganga-Brahmaputra delta and reaches Meghalaya. It is here that the orographic effect on the monsoon winds and the consequent amount of rainfall is most pronounced. Cherrapunji a small town (25° 15′ N, 91° 44′ E), located at an elevation of 1,313 m above mean sea level receives an annual rainfall of 1,102 cm major portion of which occurs from June to August.

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Till recent times, this has been considered as the highest amount of rainfall for any station in the world. But the recently recorded observations have shown that Mawsynram (25° 18′ N, 91° 35′ E) located at 1,329 m above sea level just 16 km to the west of Cherrapunji records higher annual rainfall of 1,221 cm.

Both the stations are located on the southern slopes of the Khasi hills at the northern end of a deep valley running from south to north. When the monsoon winds blow from the south, they are trapped within the funnel shaped valley and strike Cherrapunji and Mawsynram in a perpendicular direction and give copious rains.

However, the heaviest rainfall occurs when the winds blow directly on the Khasi hills. Cherrapunji and Mawsynram receive more rainfall in a day than the annual rainfall of many parts of the country. The highest records of rainfall in a day for these two stations are 103.6 cm and 99 cm respectively. The rainfall is well over 200 cm in most parts of the north eastern states. But the amount of rainfall decreases sharply on the leeward side of the Khasis. Guwahati, only 90 km from Cherrapunji gets only 161 cm of rainfall.

The second stream of the Bay of Bengal branch goes to the Himalayan foothills and after reaching there, it is deflected to the west by the size and orientation of the Himalaya and brings widespread rainfall to Indo-Gangetic plain. The rainfall by this stream is characterised by a steady decline as we move from east to west up the plain.

The eastern coastal belt, particularly in Tamil Nadu, remains relatively dry during the south-west monsoon period. This is because the Tamil Nadu coast lies in the rain-shadow area of the Arabian Sea current and is parallel to the Bay of Bengal current.

Break in the Monsoon:

During the rainy season, particularly, in July and August, there are certain periods when the monsoons become weak. The clouding decreases and rainfall practically ceases over the country outside the Himalayan belt and southeast peninsula. This is known as break in the monsoon.

The latest studies have revealed that the breaks are likely to occur more frequently during the second week of August. The normal duration of the break is about a week but on some occasions this could be longer. The longest breaks have been known to persist for two to three weeks, but such occasions are rare.

The breaks are believed to be brought about by the collapse of the Tibetan High. This results in the northward shifting of the monsoon trough. The axis of the trough lies at the foothills of the Himalayas during the break period. Even when most parts of the country have to content without rainfall during the break period, heavy rainfall occurs over the sub-Himalayan regions and the southern slopes of the Himalayas.

This leads to high flooding of the rivers having their catchment areas in the Himalayas. On an average one or two breaks do occur during the rainy season. Statistical studies of the monsoon show that in 85 out of 100 years there is a break in the monsoons.

Monsoon Depressions:

A major part of the monsoon rainfall is generated by depressions originating in the Arabian Sea but more so in the Bay of Bengal. Some depressions develop over land also. About 3-4 depressions are formed per month from June to September. Almost all of them are sucked inward through the deltas of great rivers, the Ganga, the Mahanadi, the Godavari, the Krishna and the Cauvery and cause heavy rain in these areas.

In June the depressions in the Bay of Bengal originate between 16° N and 21° N and west of 92° E. The majority of them move towards the north-west. During July and August they originate north of 18° N and to the west of 90° E and move generally in a west north-westerly direction. In September the Bay storms originate to the north of 15° N and to the west of 90° E. Majority of the cyclones move along the monsoon trough and most of them merge with the seasonal low over north-west India. (Fig. 5.17)

In the Arabian Sea the formation of depressions in June is between 17°N and 20°N. They move either in north-west or in northerly direction and may affect Gujarat or Maharashtra. Storms during August and September are rare and are formed close to Maharashtra-Gujarat coast.

Most of the rainfall in central and northern parts of the country is caused by these depressions. The satellite imagery shows thick clouds associated with these depressions. Sometimes they give excessive rainfall, as much as 60 cm in a single day, resulting in heavy loss of life and property. The absence of depressions or a change in their tracks results in deficit or no rain.